1. Technical Field
This invention relates generally to high-speed spindle assemblies for shaping a non-circular hole in a work piece.
2. Related Art
Some production applications require the formation of non-circular holes. For example, in the manufacture of pistons for an internal combustion engine, the so-called pin holes formed for the wrist, or gudgeon, pin often have a slight trumpet shape opening toward the center for accommodating flex in the wrist pin. Additionally, this trumpet shape of the pin hole is optimally designed with a slight ovality to further account for distortion in the wrist pin as the piston moves through its various cycles. This trumpet shape and non-circular cross section of the pin hole must be formed to exacting tolerances. For example, tolerances in the range of 3 to 5 microns are often required for these types of applications.
There are industrial methods for creating such non-circular holes to exacting tolerance specifications, including hydraulically actuated milling tools and electro-chemical machining techniques. The prior art has also suggested boring non-circular shapes using a special machine tool spindle equipped with active magnetic bearings. Active magnetic bearings act upon the principle that the rotating spindle is formed of, or with, a ferromagnetic material that is supported in a magnetic field generated by an electromagnet stator. A control system, with appropriate power amplifiers, modulates the magnetic field to maintain the spindle in the desired radial position while it rotates. This radial position can be maintained even under changing load conditions.
While active magnetic bearing systems provide exceptional spindle mobility, they can be expensive to produce and difficult to control. In order to achieve high-dynamic performance and acceptable levels of tool acceleration, it is necessary to provide very large, powerful magnetic bearing actuators. Furthermore, active magnet bearing systems can be difficult to dampen properly in some cutting conditions and if not properly sized and controlled.